Bismuth-Modulated Surface Structural Evolution of Pd₃Bi Intermetallic Alloy Catalysts for Selective Propane Dehydrogenation and Acetylene Semihydrogenation

Atomic regulation of metal catalysts, especially of the active surface, is key to optimizing the catalytic performance. In this work, we tuned surface Pd coordination by varying bismuth loadings in the Pd–Bi alloy system, facilitating different catalytic performances for propane dehydrogenation (PDH) and acetylene semihydrogenation model reactions. In situ X-ray absorption spectroscopy, atom-resolved scanning transmission electron microscopy combined with elemental distribution analysis, infrared spectroscopy, and in situ X-ray photoelectron spectroscopy were employed to characterize the evolution of the surface and bulk structures in Pd–Bi catalysts with changing Bi composition. At low Bi loading, the catalyst nanoparticle (NP) surface was partially transformed into the Pd–Bi intermetallic compound (IMC). The partially alloyed surface has improved catalytic performance compared with Pd NPs. At slightly higher Bi loading, a Pd core–Pd₃Bi shell structure was formed, which displayed significantly improved selectivity rate and stability. In the Pd₃Bi IMC surface structure, the adjacent Pd atoms are sufficiently far apart to give catalytically isolated active sites, which significantly enhance the selectivity (>95%) to propylene in PDH and give a higher ethylene selectivity (80%) for acetylene semihydrogenation compared with Pd NPs. At higher Bi loading, a full Pd₃Bi is formed; however, at even higher loading, an overcoating of excess BiO_x leads to a loss in activity. This work demonstrates that in intermetallic alloy catalysts, the surface and bulk structures of the NPs are different with different promoter metal loadings. Importantly, the catalyst performance is not only determined by the alloy structure but also can be significantly affected by the properties of the noncatalytic oxide promoter.